During combustion of particular fuel, usually coal, in a pressurized fluidized bed in a PFBC power plant, the bed is supplied with combustion air in the form of compressed air from the pressure vessel which surrounds a combustor, in which the fluidized bed is arranged, via fluidization nozzles below the bed. The flue gases which are generated during the combustion process pass, in the combustor, through a freeboard above the bed surface, are cleaned and supplied to a gas turbine. The flue gases drive the gas turbine, which in turn drives a generator as well as a compressor which supplies the pressure vessel with compressed air. The combustion air, the flue gases and the devices which handle these gases are included in the gas cycle of the plant. The gas cycle thus includes, among others, the gas turbine.
For cooling the bed to a temperature of about 850.degree. C., a steam generator in the form a tube bundle, which constitutes a component in a steam cycle, is placed in the bed. The designation tube bundle is used here to describe a component which is usually divided into a plurality of tube bundles. Steam is generated in the tube bundle, energy being extracted from the bed via the steam turbines to which the steam is passed in the steam cycle. At full load the entire tube bundle is within the bed. The cooling capacity of the tube bundle must be dimensioned to the power output from the bed to be able to maintain the correct bed temperature during full load operation of the plant. If the reduction of the power output is achieved by a reduced fuel supply, the power generation in the bed is decreased, while at the same time the cooling capacity of the tube bundle is unchanged, which entails an undesired reduction of the bed temperature. The cooling capacity of the tube bundle in the bed can be reduced by lowering the bed level, whereby part of the tube bundle will be located above the surface of the bed. As a consequence of this, a load change in the plant is normally controlled by reducing the cooling capacity of the tube bundle in the bed by varying the bed level, in which case a certain part of the tubes in the tube bundle may be above the bed surface, thus not cooling the bed. On the other hand, the exposed tubes cool the flue gases flowing from the bed to the gas turbine, the temperature of the flue gases thus being reduced, which entails a further addition to the power reduction in the gas turbine during partial load operation, that is to say that the steps taken to change the power output from the steam turbine lead to a corresponding change of the power output from the gas turbine.
During power control, the fuel and the air quantity supplied to the bed must be controlled in dependence on the power extracted from the bed and the bed level at the same time be controlled such that the cooling capacity of the tube bundle is adapted to the energy supply, whereby the correct bed temperature is maintained. At constant load output, the height of the bed is kept constant.
The normal way of reducing the power output to the steam turbine is to lower the bed height. A consequence of this is, as mentioned, that exposed steam tubes above the bed surface will then cool combustion gases flowing to the gas turbine, thus further reducing the gas turbine power. In this way, also the partial load efficiency of the plant is reduced.
During combustion in a plant of the kind mentioned, nitrogen compounds, NO.sub.x, are formed as a constituent in the flue gases. For environmental reasons, the aim is to considerably reduce the contents of nitrogen oxides in the flue gases. This is done, for example, by supplying ammonia to the flue gases, thus achieving reduction of NO.sub.x. This chemical process, whereby NO.sub.x in the waste gases is reduced by the addition of ammonia, proceeds optimally during full load operation of the power plant, that is, when the flue gas temperature amounts to about magnitude of 850.degree. C., at which temperature the flue gases, with a good result, are freed from the greater part of the NO.sub.x contents. During partial load operation of the plant, when the flue gases keep a lower temperature, the NO.sub.x reduction process with reduction with NH.sub.3 does not function satisfactorily, and therefore other means have to be introduced such as, for example, the use of catalysts, for the removal of nitrogen compounds in the flue gases to be completed in an acceptable way. In addition, such catalysts render the plant considerably more expensive.
It is known to increase the efficiency for the entire plant by secondary combustion in the flue gas paths to considerably raise the temperature of the hot gases supplied to the gas turbine. See, for example, EP-B-144 172. With such a secondary combustion, the temperature of the gases can be raised, for example to the order of magnitude of 1300.degree.1400.degree. C., thus creating more favorable conditions, from the point of view of efficiency, for utilizing a gas turbine. However, such a type of secondary combustion, whereby the temperature of the flue gases is raised considerably ahead of the inlet to the gas turbine, does not solve the problems which in this description are shown to be associated with partial load and the effect of measures for reduction of nitric oxides. In addition, efforts for mechanical separation of dust particles in the flue gases, for operation at the high flue gas temperature mentioned, are rendered more difficult.
During operation of a PFBC power plant, the flue gas temperature may be allowed to vary within the temperature interval of 400.degree. C.-900.degree. C. At a temperature as high as 900.degree. C., certain ash particles may melt on the surface, thus forming harmful agglomerates which disturb the function of the process. The lowest temperature limit mentioned is not very favorable either, since, among other things, the performance of the gas turbine is then greatly reduced.
It would be desirable to achieve a constant temperature of the flue gases leaving the combustor, both during full load and partial load operation.
For reasons of compromise a nominal working temperature of around 850.degree. C. is chosen for both the bed and the flue gases, for which working temperature the components in the plant are dimensioned. This compromise is based on a balancing based on conditions relating to combustion in the bed and on creating as high a temperature as possible for the gases which are distributed to the gas turbine. It would be favorable to reduce the temperature of the bed by some 10 or 30 degrees centigrade. When, for example, coal is used as fuel, alkali metals bound in the coal are evaporated and deposited on surfaces in the flue gas walls, for example on blades in the gas turbine. In addition, certain types of coal contain a greater percentage of such alkali metals, so this problem is accentuated when using such coal during the combustion. Also the flue gas temperature could advantageously be maintained higher, for example some 10 to 30 degrees centigrade, than the chosen working temperature (850.degree. C.) without cleaning processes in the flue gas paths being disturbed.
When designing a PFBC power plant, this is dimensioned for full load operation at maximum air flow through the combustion process. Since the density of the air is greatest at the lowest presumptive exterior air temperature at the site of the plant, the plant will consequently perform full load only at this lowest exterior air temperature. In principle, this means that the plant is operated under partial load conditions as soon as the exterior air temperature deviates from the lowest temperature for which the plant has been designed. In the following description, the word partial load also includes such unintentional partial load operation as is caused by raised exterior air temperatures.
The publication EP 363812, A2, demonstrates for a plant of AFB type, that is, a plant with a fluidized bed at atmospheric pressure, a method for reducing the relatively high percentage of CO gas, generated in the bed in this type of plant, by converting CO to CO.sub.2 during a combustion of CO with the aid of additional air. To allow this combustion to take place with sufficient speed, the working temperature in a space above the bed is raised by supplying extra fuel above the bed.
The combustion in the bed in the described AFB plant takes place, for the purpose of reducing the generation of NO.sub.x gases, under sub-stoichiometric conditions, that is, the combustion of the fuel in the bed takes place when there is a deficiency of air, which in turn causes a high percentage of CO in the waste gases from the bed. Further, it should be emphasized that the combustion of the additional fuel according to the known technique described in the publication is not performed downstream of steam generating and superheating tubes, whereby another object of the extra combustion is to maintain high temperatures to the steam tubes which are located downstream of the region for combustion of the additional fuel.
In a PFBC power plant the combustion in the fluidized bed takes place at high pressure and under superstoichiometric conditions, that is to say that air is supplied to the bed in such quantity that there is excess air in all parts of the bed where metal objects, for example boiler tubes, are present. This excess air is necessary for the formation of erosion-resistant oxide layers on the metal surfaces, but also for achieving as complete a final combustion of the bed fuel as possible. Under normal operating conditions, very small amounts of CO are then formed, and therefore no extra combustion with air supplied above the bed is necessary.
The excess air which is supplied to the bed in a PFBC plant in turn causes a relatively high percentage of NO.sub.x gases in the flue gases from the bed, which NO.sub.x gases must be removed, preferably within a temperature interval around the normal working temperature for the freeboard.